To preserve the environment, one type of an image forming apparatus prints an image on a sheet with a decolorizable color material. Such a decolorizable color material can be decolorized when heated to a certain temperature. Therefore, a sheet from which the image formed of the decolorizable color material has been erased can be reused. In some cases, for example, when forming a full-color image on a sheet, an image forming apparatus forms the image with plural layers of color materials, and some or all of the color materials may be decolorizable.
However, depending on the environmental conditions for the printing, a density of the printed image, the number of times the sheet has been reused, and the type of the sheet, a part of the printed image may be left after an erasing process. Especially, when the image is formed on a sheet with plural layers of color materials, as described above, and a layer of the decolorizable color material is formed underneath the other layers, the color of the decolorizable material may not be sufficiently decolorized. This is because the decolorizable color material does not reach the decolorizable temperature when the sheet is heated.